Might be able to get guardianship info & forms online for your state, and/or contact the county for help..
For the school issue, maybe something is happening there, or he is being abused or harassed?
Contact a specialist for an updated autism assessment & evaluation & for the non eating , school issues and future adult hood, self reliance.
We don't have an Autism forum, our sister site does, you may find more help there -
Psych Central - Trusted mental health, depression, bipolar, ADHD & psychology information
Maybe he would do better to finish his schooling another way.
Online learning - HS or GED.
graduate high school online free - Google Search
ged online - Google Search